Water treatment process and plant using ballasted flocculation and settling

ABSTRACT

A method for treating wastewater using a ballasted flocculation technique includes continuously measuring the concentration of suspended solids, organic matter or other impurities in the water to be treated prior to directing the water to be treated to a flocculation tank. Based on this measurement, the amount of ballast necessary to obtain treated water of a predetermined quality is then calculated. In the flocculation tank, ballast and a flocculating reagent are added to the water to form a water-floc mixture. The water-floc mixture is directed to a settling tank where a sludge-ballast mixture is settled. The sludge-ballast mixture is directed to a mixing tank and then to a separator to separate the ballast from the sludge. The separated ballast is directed to the flocculation tank. The separated sludge is directed to the mixing tank when the level of sludge-ballast mixture in the mixing tank is lower than a predetermined level.

This application is a U.S. National Stage application of PCT ApplicationNo. PCT/EP2007/064547, with an international filing date of Dec. 26,2007. Applicant claims priority based on French application serial. no.06/11562 filed Dec. 29, 2006. Both applications are incorporated hereinin their entirety.

This invention relates to the field of water treatment.

Water treatment, primarily the potabilisation of water and the treatmentof urban or industrial wastewater to be depolluted, frequently uses aprocess consisting of coagulating the water filled with impurities to betreated with a coagulant often constituted by a trivalent metal salt,flocculating the coagulated water with a flocculating reagent usuallyconstituted by an organic polymer, and settling the floc formed in asettling tank, the sludge being partially extracted from the bottomportion of the settling tank, and the treated water being extracted fromthe top portion of the settling tank.

Such a technique makes it possible to remove the dissolved or suspendedcolloidal impurities constituted by organic matter, micropollutants andmicroorganisms, in particular, from the treated water.

The invention more specifically relates to technology referred to asflocculation-settling with ballasted floc, which implements a ballastconstituted by a fine and dense granular material such as microsand, forexample, injected into the flocculation zone, in order to increase thespeed of floc formation, serving as a flocculation initiator, and alsoto increase, by increasing the density, the rate of settling of the flocformed during the flocculation phase, which makes it possible to reducethe size of the structures.

In practice, the coagulated water is placed in contact in a reactor witha flocculating reagent, such as a polymer, and a ballast, and closelymixed therewith by agitation means. The time of contact between thewater with the flocculant and the ballast must be sufficient to allowfor the formation and increase in size of the floc constituted by theaggregated impurities by means of the flocculant around the ballast.

Microsand, with a mean diameter of between around and 400 micrometers,and usually 80 and 300 micrometers, is the ballast used most often forreasons of availability and cost.

The ballasted flocculation-settling technology is described inparticular in the following patents:

-   -   patent application FR-A-2627704, published on 1 Sep. 1989;    -   patent application FR-A-2719234, published on 3 Nov. 1995.

In this technology, the ballast is usually separate from the sludgeextracted from the settling structure so as to be capable of beingrecycled, by recirculation means, in the process.

During the recycling, a small portion of the ballast goes with thesludge. It is therefore necessary to periodically inject new ballastintended to compensate for the loss of ballast.

It is important to control the loss of ballast with the sludge in orderto minimise the amount spent on new ballast. In addition, excessiverecirculation can cause a deterioration in the quality of the sludgeextracted, i.e. it can result in excessively-diluted extracted sludge,corresponding to “water losses”.

To minimise these losses, the separation of the ballast from the sludgein order to recycle said ballast in the process is generally performedby hydrocyclone separation of the sludge/ballast mixture.

However, the risks of hydrocyclone malfunction increase rapidly beyond agiven solids content in the underflow (often around 40% solid byvolume).

Finally, significant ballast losses may take place when the underflow ofthe hydrocyclone is clogged, and the ballast then goes in overflow.

In an attempt to solve these problems, the prior art, namely patentapplication WO-A-03053862, published on 3 Jul. 2003, suggests pumpextracting the mixture of sludge and ballast from the bottom portion ofthe settling tank and routing it to an agitated intermediate mixingzone, extracting the mixture of sludge and ballast present in saidintermediate mixing zone and subjecting it to a step of sludge/ballastseparation by hydrocyclone separation, and recirculating a portion ofthe sludge by adjusting the rate of said recirculation.

However, the implementation of this technique quickly leads to adeterioration in the quality of the treated water, if it is desirable toimplement the step of separation of the ballast and the sludge underconditions leading to the recycling, in the flocculation zone, of thecleanest possible ballast. Indeed, the influx of additional solids tendsto pollute the quality of the treated water.

In any case, this process described in WO-A-03053862, as well as thosedescribed in FR-A-2627704 and FR-A-2719234 do not make it possible tooptimise the amounts of ballast to be implemented according to the loadof material to be flocculated from the water to be treated. Thisoptimisation would make it possible simultaneously to:

-   -   perform the flocculation of impurities to be removed,    -   minimise the ballast losses,    -   reduce the water losses,    -   obtain high-quality treated water,

without considerably increasing the energy needed for the formation offloc and for the recirculation of sludge.

The objective of this invention is to provide a technique making itpossible to approach or to achieve such an optimisation.

This objective is achieved by the invention, which relates to a processfor treating water filled with dissolved or suspended colloidalimpurities, in a treatment plant, including steps consisting of:

-   -   contacting, in a flocculation zone, said water, at least one        ballast constituted by at least one insoluble granular material        that is heavier than water, and at least one flocculating        reagent so as to enable the floc to form;    -   introducing the mixture of water and floc thus formed into a        settling zone;    -   separating the water treated in the top portion of said settling        zone from a sludge and ballast mixture in the bottom portion of        said settling zone and routing it to an agitated intermediate        mixing zone;    -   extracting the mixture of sludge and ballast present in said        intermediate mixing zone and subject it to a step of        sludge/ballast separation by hydrocyclone separation,    -   recycling the underflow of the hydrocyclone separation step in        said flocculation zone;    -   extracting a portion of the sludge coming from the overflow of        the hydrocyclone separation step and recirculating the other        part of said sludge in said agitated intermediate mixing zone;

characterised in that it includes:

-   -   a step consisting of continuously measuring at least one        parameter representing the concentration of impurities in the        water before or when it enters said flocculation zone;    -   a step consisting of using the results of said measurement thus        performed so as to continuously deduce the amount of ballast        that needs to be implemented to obtain treated water of a        predetermined quality.

Such a process makes it possible to know, at any time, the amount ofballast needed according to the pollutant load to obtain theflocculation of the entire pollutant load while minimising the losses ofballast.

The process according to the invention preferably includes:

-   -   a step consisting of continuously measuring the ballast        concentration in the mixture extracted from said settling zone        or in the mixture present in said flocculation zone;    -   a step consisting of deducing, from the continuous measurement        of the concentration of mixture extracted from said settling        zone, the concentration of ballast actually present in said        plant;    -   a step consisting of refilling the flocculation zone with        ballast when said concentration of ballast actually present in        said plant is lower than a predetermined threshold.

According to an advantageous alternative, said step consisting ofrefilling the flocculation zone with ballast when said amount of ballastactually present in said plant is lower than a predetermined thresholdis performed automatically.

The process also advantageously includes a step consisting of using theresults of said measurement of said at least one parameter representingthe concentration of impurities in the water so as also to deduce theamount of said flocculating reagent that needs to be dispensed into theflocculation zone in order to obtain treated water having saidpredetermined quality.

According to an alternative, the process also includes a preliminarystep consisting of continuously injecting into said water, before itenters the flocculation zone, at least one coagulating reagent accordingto a predetermined amount, and, as the case may be, at least one reagentintended to adjust its pH according to a predetermined amount. This stepof injecting the coagulating reagent and adjusting the pH is verygenerally necessary. However, there are occasional cases in which it isnot absolutely essential, in particular when the water to be treatedcontains very little organic matter.

Said parameter representing the concentration of impurities in the waterused to implement the proposed process is preferably the concentration(X) of the so-called “overall SS” or overall suspended solids in thewater, with the so-called “overall SS” concentration being calculated bytaking into account all or some of the following parameters:

-   -   the concentration in said water of suspended solids,    -   the concentration in said water of organic matter,    -   the concentration of microorganisms in the raw water,    -   the concentration of micropollutants in the raw water,    -   said predetermined amount of coagulating reagent,    -   said predetermined amount of reagent intended to adjust the pH.

According to an alternative of the process disclosed, the amount ofballast that needs to be suspended in the flocculation tank in order toobtain treated water of said predetermined quality is determined on thebasis of the concentration Y of ballast necessary for said so-called“overall SS” concentration X calculated according to the formula I:Y=aX^(b)+c (in which a is between 0.4 and 1, b is between 0.3 and 1 andc is between 0 and 2) and on the basis of the approximate volume ofwater present in said plant.

The process proposed preferably includes a step also consisting ofcontinuously adjusting the amount of said flocculating reagent dispensedinto said flocculation zone according to said concentration of ballastthat needs to be implemented in order to obtain treated water of apredetermined quality.

Said step consisting of extracting the mixture of sludge and ballastfrom the bottom portion of said settling zone and routing it to anintermediate mixing zone is preferably performed using at least oneendless screw. Such an endless screw enables much more regular routingof the mixture of sludge and ballast coming from the bottom portion ofthe settling tank to the intermediate zone, which the use of a simplepipeline with a pump would not allow.

According to an advantageous alternative, the process also includes astep consisting of adjusting the flow rate of the sludge coming from theoverflow of the hydrocyclone separation step, recirculated to saidintermediate mixing zone so as to maintain a predetermined level ofsludge and ballast in said intermediate mixing zone.

In such a case, the process also preferably includes a step consistingof storing the sludge coming from the overflow of the hydrocycloneseparation step in a tank having an overflow pipe, measuring the levelof sludge and ballast mixture present in the intermediate mixing zone,and releasing at least some of said sludge in the tank into saidintermediate mixing zone when the measured value is lower than apredetermined threshold.

According to a preferred aspect of the technique disclosed, said step ofhydrocyclone separation of the sludge and ballast mixture coming fromsaid settling zone is performed by implementing an injection ofadditional liquid tangentially to said sludge.

Said additional liquid is injected in an amount corresponding to 5 to100% by volume, and typically 5 to 20% of the volume of the sludge andballast mixture introduced in the hydrocyclone separation step.

The use of such an additional liquid makes it possible to obtain acleaner ballast in the hydrocyclone underflow, essentially free of thegangue of impurities surrounding it when it enters the hydrocyclone.

According to an alternative of the process described, said stepconsisting of contacting, in a flocculation zone, said water, at leastone ballast constituted by at least one insoluble granular materialheavier than water, and at least one flocculating reagent in order toallow floc to form, includes:

-   -   a step consisting of defining, in the flocculation zone, by        means of a fully immersed flow-guide structure, an internal zone        in which, by agitation, a turbulent axial flow is created of the        mixture of the water to be treated, the ballast and the        flocculant in an axial direction of said flow-guide structure,    -   a step consisting of injecting said flocculating reagent by        means of a hydraulic distribution device in said axial flow,    -   a step consisting of distributing this flow, by means of a        static device opposing the rotation of said flow and arranged at        the outlet of this flow-guide structure;    -   a step consisting of allowing said mixture to circulate in a        peripheral zone surrounding said flow-guide structure, in an        opposite direction up to the inlet of said internal zone; and    -   a step consisting of moving said mixture to said settling zone.

The use of agitation means provided in the internal zone defined by theflow-guide structure allows for an intense mixture of the ballast withthe flocculant and the suspended solid involved in a good formation offloc in the peripheral zone. The partition of the flocculation zone intoan internal zone and a peripheral zone makes it possible to prevent themechanical destruction of this floc by the agitation means from whichthe floc is protected by the flow-guide structure.

The process preferably includes a step consisting of converting the flowleaving said flow-guide structure into an axial flow by means of aflow-distributing static device. This device can be separate from theflow-guide structure, for example attached to the base of theflocculation zone. However, the flow-distributing static device willpreferably be provided within the actual flow-guide structure.

The process described makes it possible to provide a contact timebetween the water to be treated, the flocculating reagent and theballast in said flocculation zone, from one to several minutes.

Said ballast is preferably microsand with a mean diameter of betweenaround 20 and 400 micrometers.

According to an alternative of the process, a granular material withadsorption properties, such as active carbon powder, or a granularmaterial with ion or molecule exchange properties, such as a resin, isintroduced into the flocculation zone or upstream of the flocculationzone so as to allow for a sufficient contact time of said material withthe water to be treated.

This material may, where appropriate, constitute said ballast or asecond ballast.

The settling step of the process is preferably a lamellar settling step.

The invention also relates to any plant for implementing such a process,including:

-   -   at least one flocculation tank provided with at least one        agitator;    -   a pipeline supplying water to be treated in said flocculation        tank;    -   a settling tank provided with an outlet for the treated water in        the bottom part;    -   a pipeline connecting the bottom portion of the settling tank to        an intermediate tank provided with at least one agitator;    -   a pipeline connecting said intermediate tank to a hydrocyclone;    -   a pipeline for recirculation of some of the overflow of the        hydrocyclone to said intermediate tank;

characterised in that it includes:

at least one first sensor intended to continuously measure at least oneparameter representing the concentration of impurities in the waterentering said plant;

a calculator making it possible to continuously deduce, frommeasurements taken by said first sensor, the amount of ballast that mustbe implemented in order to obtain treated water of a predeterminedquality.

Such a plant preferably includes at least one second sensor provided insaid flocculation tank or in said intermediate tank or at the level ofsaid pipeline connecting said intermediate tank to said hydrocyclonemaking it possible to continuously measure the ballast concentration inthe mixture passing through one of them, and in that said calculatormakes it possible to continuously deduce, from the measurements taken bysaid second sensor, the amount of ballast actually present in saidplant.

Such a plant advantageously includes an automatic ballast-refillingdevice.

Said calculator is preferably designed to continuously deduce, from themeasurements taken by said first sensor, the amount of flocculatingreagent that needs to be implemented in order to obtain treated water ofsaid predetermined quality.

According to an alternative, the plant also includes an automaticflocculating reagent dispenser connected to said calculator.

Said first sensor is preferably a sensor measuring the concentration inthe raw water of suspended solids and/or the concentration in the rawwater of organic matter such as the concentration of total organiccarbon in said water.

According to an alternative, the plant includes at least one sensorintended to continuously measure the quality of the water treated bysaid plant. In other alternatives, this measurement can be performedonly from time to time, and manually, as the case may be.

Said pipeline supplying water to the flocculation tank is preferablyequipped with a static mixer for mixing the water to be treated with acoagulating reagent.

Said discharge of the treated water from said settling tankadvantageously includes at least one spillway or a perforated tube.

Said settling tank is preferably equipped at its inlet with asiphoniform partition.

Also preferably, said settling tank is equipped at its inlet with asplitting structure. The latter advantageously includes at least twoplates parallel to one another arranged on each side of a submergedspillway provided between the flocculation tank and the settling tank,and plates arranged parallel to one another between the other plates,overlapping said submerged spillway, said transverse platesconstituting, with the other plates, the same number of flow channelsbetween the flocculation tank and the settling tank.

According to an advantageous alternative, said pipeline connecting thebottom portion of the settling tank to said intermediate tank isequipped with an endless screw.

According to another alternative, the plant includes a tank providedwith an overflow pipe provided on said pipeline for recirculation ofsome of the hydrocyclone overflow to said intermediate tank, andpreferably a valve provided on said pipeline for recirculation of someof the hydrocyclone overflow to said intermediate tank, said valve beingprovided downstream of said tank.

Also preferably, the plant includes a sensor that senses the level ofthe sludge and ballast mixture present in said intermediate tank.

According to an alternative, the hydrocyclone used in the plantdescribed has a cylindrical portion provided with at least onetangential supply of the sludge and ballast mixture to be treated and aconical portion, and, at the outlet of the conical portion, anadditional liquid injection chamber having a tangential supply ofadditional liquid.

According to an alternative, the plant proposed includes at least onedispenser of a material having ion or molecule exchange or adsorptionproperties in said flocculation tank.

Said settling tank is preferably a lamellar settling tank provided withvertical blades according to one alternative.

According to an advantageous feature, said flocculation tank preferablycomprises a flow-guide structure, open at its two ends and arranged at adistance from the base of said tank defining a median zone equipped withsaid agitator and a peripheral zone and a static device distributing theflow leaving said flow-guide structure. This flow-guide structure ispreferably a tube with a circular cross-section arranged vertically at adistance from the base of said flocculation tank.

According to an advantageous alternative, such a static device isintegrated in the lower portion of the flow-guide structure, preferablyat least 200 mm below the agitator. Said device is advantageouslyconstituted by at least one diametral plate of height H defining atleast two compartments. Said compartments formed by said at least oneplate preferably have surfaces that are substantially equal, and thetheoretical width B of each compartment is chosen so that there is aratio H/B between the height H of said at least one plate and saidtheoretical width B between around 1 and around 2, typically equal toaround 1.5.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, as well as the various advantages that it has, can bemore easily understood from the following detailed description of twopreferred embodiments provided in reference to the figures, in which:

FIG. 1 shows a diagrammatic cross-section view of a first embodiment ofa plant according to this invention;

FIG. 2 shows an upper perspective view of the flow-distributing deviceprovided at the outlet of the flow-guide tube of the plant shown in FIG.1;

FIG. 3 shows a lower partial perspective view of the splitting structureprovided on the submerged spillway between the flocculation tank and thesettling tank of the plant shown in FIG. 1;

FIG. 4 shows a cross-section view of the hydrocyclone of this plant;

FIG. 5 shows a diagrammatic cross-section view of a second embodiment ofa plant according to this invention;

FIG. 6 shows a cross-section view of the flow-guide tube and itsintegrated flow distributor of the plant shown in FIG. 5;

FIG. 7 shows a cross-section view AA′ of said flow distributor;

FIGS. 8 and 9 show cross-section views of other embodiments of a flowdistributor.

In reference to FIG. 1, the embodiment example described here has aflocculation tank 1 provided with a mechanical agitator 2. Thismechanical agitator 2 includes a rotatably-mounted vertical pin plunginginto the tank and equipped with blades at its ends.

The flocculation tank 1 in the preferred embodiment described here hasan essentially parallelepiped shape, but can have other shapes, inparticular circular, in other embodiments.

This flocculation tank 1 is equipped, in its central portion, with aflow-guide structure constituted by a cylindrical flow-guide tube 3receiving the agitator 2. Said flow-guide tube 3 is provided at adistance from the base of the tank and defines therein an centralinternal zone 1 a, constituted by the hole of the flow-guide tube 3, anda peripheral zone 1 b, between the external wall of said flow-guide tube3 and the lateral walls 1 c of the flocculation tank 1.

This flocculation tank 1 is also equipped, at the outlet of theflow-guide tube 3, and at a distance therefrom, with a staticflow-distributing device 4 attached to its bottom wall 1 d.

This static device 4 is shown in perspective in FIG. 2. As can be seenin the figure, it is constituted by the association of two plates 4 aand 4 b together forming a cross.

It is noted that in other embodiments, this static device can beprovided at the outlet of the flow-guide tube, but integrated thereininstead of being provided at a distance therefrom and attached to thebase.

The plant described in FIG. 1 also includes a pipeline 5 supplying waterto be treated to the flocculation tank described above, said pipeline 5joining the lower portion of said tank.

This pipeline 5 is equipped with a coagulating reagent injection means6, such as an injector, a coagulating reagent, for example, ironchloride, a pH adjusting injection means 7, such as an injector, areactor enabling the pH to be adjusted, for example lime, and a staticmixer 8 making it possible to mix the reagents brought to the pipelineby means 6 and means 7 with raw water so as to obtain, at the inlet ofthe flocculation tank, coagulated water with a predetermined pH.

The plant described in FIG. 1 also includes a ballast dispensing means9, such as a dispenser, a granular material constituting a ballast, suchas microsand, in the flocculation tank 1, and a flocculating reagentdispensing means 10, such as a dispenser, and a flocculating reagent,such as, for example, a polymer, in this same flocculation tank.

More specifically, the means 10 make it possible to dispense theflocculating reagent inside the flow-guide tube 3 in a portion thereoflocated below the blades of the agitator 2.

The plant also includes a lamellar settling tank 11, provided downstreamof the flocculation tank. In the context of this preferred embodiment,to enhance the compactness of plant, the settling tank has a wall 1 cshared with the flocculation tank, said shared wall being equipped witha submerged spillway 16 equipped with a splitting structure 17. Thesettling tank 11 is equipped with a siphoniform partition 18 forming,with said submerged spillway 16 and said splitters, a passage 18 abetween the flocculation tank 1 and the settling tank 11.

This passage 18 a is described in greater detail in reference to FIG. 3.

As can be seen in FIG. 3, the wall is shared by the flocculation tank 1and the settling tank 11 is equipped in its upper portion with asubmerged spillway 16. Said submerged spillway 16 is equipped with asplitting structure 17. More specifically, this structure is constitutedby two parallel plates 17 a arranged on each side of the submergedspillway 16, and parallel plates lib arranged between the other plates17 a, overlapping the submerged spillway 16. These transverse plates 17b constitute, with plates 17 a, the same number of flow channels betweenthe flocculation tank 1 and the settling tank 11. These channelscommunicate with the passage 18 a, which is defined by the siphoniformpartition provided in the settling tank 11.

In reference to FIG. 1, the settling tank 11 of the plant is equipped inits lower portion with a rotating device 12 for scraping sludge, and inis upper portion with horizontal blades 13.

The prior art indeed proposed tilting the blades of the lamellarsettling tanks used in the context of water treatment plants usingballasted floc so as to promote the settling of the floc. However, theinventors have discovered that the feature whereby verticals blades areprovided in the settling tank did not adversely affect the settling ofthe floc and had the advantage of facilitating the handling of theseplates. It is noted, however, that in other embodiments, the settlingtank may have tilted blades or be blade-free.

The settling tank 11 has, in its bottom portion, a discharge channel 14for sludge decanted therein, and, in its upper portion, a dischargeoutlet 15 for the treated water, constituted in this embodiment by asimple spillway. The treated water coming from this spillway is pickedup by a pipeline 15 a on which a sensor 44 is provided, making itpossible to take continuous or periodic measurements of one or moreparameters relating to the quality of the treated water. It is notedthat in other embodiments of the invention, such measurements relatingto the quality of the treated water can be obtained manually.

Still in reference to FIG. 1, the plant according to this invention alsoincludes a so-called “intermediate tank” or a mixing tank 19, equippedwith an agitator 20 constituted by a rotating pin on which the bladesare mounted.

In this embodiment, for reasons of compactness, this intermediate tank19 is attached to the flocculation tank 1. However, the base of thisintermediate tank 19 is at a lower level than that of the flocculationtank 1.

The plant shown in FIG. 1 also includes a pipeline 21 connecting thechannel 14 of the settling tank 11 to the inside of the intermediatetank 19. This pipeline 21 is equipped with an endless screw 22, therotation of which is controlled by a motor 23.

The plant also includes a pipeline 25 provided with a pump 28 connectingthe intermediate tank 19 to a hydrocyclone 26 of which the underflow 27is provided above the flocculation tank 1.

The overflow 29 of the hydrocyclone 26 is connected to a recirculationpipeline 30 joined above the intermediate tank 19. A tank 31 is providedon this recirculation pipeline 30 and equipped with an overflow pipe 32as well as a discharge pipeline 33 for this overflow pipe. The portionof the pipeline 30 provided downstream of this tank 31 is equipped witha valve 34.

The intermediate tank 19 is also equipped with a sensor 43 that sensesthe level of the sludge and ballast mixture present in the tank 19. Thissensor 43 is connected to the valve 34.

According to this preferred embodiment, the plant also includes apipeline supplying service water 35 to the underflow 27 of thehydrocyclone. This pipeline is equipped with means 10 a for supplyingthe flocculating reagent, making it possible to optimise the mixture ofthe latter with the ballast. The hydrocyclone is shown in greater detailin the cross-section of FIG. 4.

In reference to FIG. 4, the hydrocyclone 26 includes a cylindricalportion 50 provided in its upper portion with a tangential supply ofsuspension to be treated. This tangential supply is connected to therecirculation pipeline 25.

The hydrocyclone 26 also includes a conical portion 52 extending fromthe cylindrical portion 50, and which communicates with a cylindricalchamber 53. The cylindrical chamber 53 has a tangential supply 54 thatcommunicates with the service water supply pipeline 35 mentioned above.The cylindrical chamber 53 communicates with the underflow 27 of thehydrocyclone. The overflow 29 of the hydrocyclone is provided in theupper portion of the cylindrical portion 50.

In reference to FIG. 1, the plant described includes sensors 40, 40 aintended to continuously measure parameters representing theconcentration of impurities in the raw water to be treated, entering theflocculation tank 1. These impurities may be of different types and/orin different forms (suspended solids, colloidal matter, dissolvedmatter, microorganisms, micropollutants, and so on). The parametersmeasured can be, for example, the concentration of suspended solids inthe raw water or the concentration of organic matter in said raw water,measured as the TOC (total organic carbon), or the UV absorbency at 254nm or the COD (chemical oxygen demand) or the oxygenconsumed-permanganate (KmnO₄) or any other measurement making itpossible to accurately estimate the OM (in particles or dissolved).

As will be explained below in greater detail, the measurement of theseparameters, taken by sensors 40, 40 a will be used to deduce theso-called “overall SS” concentration of the water entering theflocculation tank 1.

It is noted that in the embodiment described in FIG. 1, these sensors40, 40 a are provided upstream of the static mixer 8 provided on thepipeline 5 for supplying water to be treated to the flocculation tank 1.The measurements taken by the sensors 40, and 40a are therefore done onraw water. However, it is also possible to envisage in otherembodiments, taking measurements on the coagulated water, and thereforeplacing a corresponding sensor downstream of the water coagulationmeans.

The plant described also includes a sensor 41 provided at the level ofthe pipeline 25 connecting the intermediate tank 19 to the hydrocyclone26. This sensor 41 makes it possible to continuously measure the ballastconcentration (in the context of this embodiment example, microsand) ofthe mixture of ballast and sludge passing through this pipeline 25. Sucha ballast concentration corresponds to the ballast concentration presentin the intermediate tank 19 and is proportional to the ballastconcentration of the mixture of water, ballast and polymer present inthe flocculation tank 1.

It is also noted that in other embodiments, this ballast concentrationsensor can be provided either in the intermediate tank 19 or in theflocculation tank 1.

The plant also includes a calculator 42 making it possible to collectthe measurements taken by sensors 40, 40 a and 41.

The operation of the plant described above in reference to FIGS. 1 to 4is as follows.

Raw water to be treated arrives by the pipeline 5. Coagulating reagent(for example, iron chloride) and reagent intended to adjust the pH (forexample, lime) are injected in predetermined amounts into this rawwater, by means 6 and 7 respectively, and mixed therewith by the staticmixer 8 so that the water arriving in the flocculation tank 1 iscoagulated and has an optimised pH according to the type of coagulantchosen.

The coagulated water arriving in the lower portion of the flocculationtank 1 follows an ascending path into the peripheral zone 1 b thereofdefined by the lateral walls 1 b of the flocculation tank 1 and theexternal walls of the flow guide tube 3 (as show in FIG. 1 by the arrowspointing upward in the flocculation tank 1), before penetrating saidflow-guide tube 3 through its upper opening and following a descendingmovement into the central internal zone 1 a defined by the hole of saidflow guide tube 3 (as shown in FIG. 1 by the arrows pointing downward inthe flow guide tube 3).

It is noted that in other embodiments, the coagulated water may arrivein the upper portion of the flocculation tank, with its flow thenfollowing a descending movement into the peripheral zone and anascending movement into the internal zone.

When it descends into the flow-guide tube 3, the coagulated waterfollows a descending movement with a horizontal radial component due tothe movement of the blades of the agitator 2.

As it leaves the flow-guide tube 3 through the lower outlet thereof, theflow reaches the static flow-distributing device 4. This staticflow-distributing device 4, by its configuration, makes it possible tostrongly attenuate the radial component of the flow coming from theflow-guide tube 3 and to distribute this flow in a substantially evenmanner in the entire peripheral zone 1 b of the flocculation tank 1.

With the ballast dispensing means 9, an amount of ballast, calculated asindicated below, is dispensed into the water present in the flocculationtank 1.

With the flocculating reagent dispensing means 10, an amount offlocculating reagent, calculated as indicated below, is continuouslydispensed inside the flow-guide tube 3 in the same water.

With the flow-guide tube 3 and the agitator 2, the mixture of saidpolymer and the microsand with the water is optimised.

In the flocculation tank 1, floc constituted by ballast is formed, andthe impurities contained in the water agglutinate around said floc,owing to the flocculating reagent. The improvement of the mixture ofwater and ballast and the flocculating reagent makes it possible tooptimise the formation of floc as well.

With the sensors 40 and 40 a, respectively, the suspended solids contentand the concentration of organic matter (OM) in the raw water aremeasured continuously.

The corresponding measurements are sent to the calculator 42, whichcombines these measurement of the data concerning the predeterminedamounts of coagulating reagent and the reagent intended to adjust the pHused and the concentration of microalga in the raw water so as to deduceor determine a concentration of so-called “overall SS” in the coagulatedwater entering the flocculation tank 1, representing the concentrationof pollutants to be removed, contained in said water.

The calculator 42 then calculates the amount of flocculating reagentthat needs to be implemented in the plant by means 9, according to:

-   -   the flow rate of the raw water to be treated arriving in the        plant through the pipeline 5,    -   said so-called “overall SS” concentration in the water arriving        in the flocculation tank 1,    -   the particle size of the granular material constituting the        ballast.

The calculator 42 calculates the amount of ballast that needs to beimplemented in the plant in order to obtain treated water of apredetermined quality, said amount corresponding to a minimum ballastconcentration in the flocculation tank.

In the context of this embodiment example, said ballast concentration Yis calculated by the calculator 42 using the formula:Y=0.4208×X ^(0.3667)

in which X corresponds to the “overall SS” concentration.

In other embodiments, other methods for calculating this ballastconcentration can be envisaged.

After having passed through the flocculation tank 1, the mixture formedby water and floc enters the settling tank 11 by passing over thesubmerged spillway 16.

During this passage, said mixture passes through channels defined byplates 17, 17 a of the splitting structure described in reference toFIG. 3.

This structure makes it possible to obtain, at the outlet of saidchannels, a flow of the mixture of water and floc that is betterdistributed over the length of the submerged spillway 16.

This mixture then passes through the passage 18 a defined by thesubmerged spillway 16 and the siphoniform partition 18 to arrive in thesettling tank 11.

In the settling tank 11, the floc formed by matter aggregated around theballast settles and accumulates on the bottom wall of the settling tank11 to form a mixture of sludge and ballast. This settling is improved bythe presence of blades 13 provided in the upper portion of the settlingtank 11.

The rotating scraping device 12 makes it possible to direct this mixtureof sludge and sand into the channel 14 of the settling tank 11.

The treated water free of its impurities is discharged into the upperportion of the settling tank 11 by the discharge outlet 15.

The mixture of sludge and ballast present in the channel 14 of thesettling tank 11 is extracted from said channel 14 through the pipeline21 by means of the endless screw 22 provided in the pipeline 21, whichendless screw is actuated by the motor 23.

This sludge and ballast mixture is routed at a primarily constant speedinto the intermediate tank 19. This routing is facilitated by the factthat the base of the tank 19 is provided at a level lower than that ofthe flocculation tank 1, which enables the pipeline 21 to pass below it.

The sludge and ballast mixture is mixed by agitation means 20 providedin the tank 19, in which it follows an ascending, then descendingmovement (as indicated by the arrows pointing upward and downward).

This sludge and ballast mixture is continuously extracted from theintermediate tank 19 through the pipeline 25 by means of the pump 28 soas to be routed toward the hydrocyclone 26 intended to separate theballast from the sludge contained in said mixture.

This separation is improved by the injection, through the pipeline 25,of service water into the hydrocyclone. This injection of service watermakes it possible to obtain, in the underflow 27 of said hydrocyclone26, a ballast that is essentially free of organic matter. The ballastrecovered in the underflow of the hydrocyclone 27 is redistributed inthe flocculation tank 1.

The overflow 29 of the hydrocyclone 26, constituted by diluted sludge,is routed through the pipeline 30 to the tank 31 provided thereon. Saiddiluted sludge accumulates in said tank 31. A part of said dilutedsludge is discharged through a pipeline 33 connected to an overflow pipe32 provided in said tank, while another part is rerouted by way of thepipeline 30 to the intermediate tank 19. However, this recirculation isimplemented only when the level of mixture present in the tank 19 anddetected by the sensor 43 is lower than a predetermined value. In thiscase, the valve 34 provided on the pipeline 30 opens to release some ofthe contents of tank 31 into tank 19 and closes when the sensor 43detects that the level of mixture in said tank 19 has reached thepredetermined value.

This mechanism makes it possible to maintain an essentially constantballast concentration in the tank 19.

The sensor 41 continuously measures this concentration and transmits itto the calculator 42, which deduces the amount of ballast actuallypresent in the plant.

If this amount is lower than or excessively lower than the amount ofballast needed to obtain a predetermined water quality (amountcalculated as indicated above from the ballast concentration Y), inother words, if this amount falls below a predetermined value, thecalculator 42 commands the dispensing means 9 to automatically fill theplant with an additional amount of ballast so as to obtain saidnecessary amount.

A second embodiment example is shown in reference to FIGS. 5 to 7.

The plant described in FIG. 5 is completely identical to that shown inFIG. 1, except that:

-   -   its flocculation tank 1 is equipped with a flow-guide tube 3 a        including a flow distributor 4 a;    -   and in that it includes a dispenser 9 a of a second granular        material (material having adsorption properties, or ion or        molecule exchange properties) in the flocculation tank 1.

In reference to FIG. 6, the flow distributor included in the tube ispositioned more than 200 mm below the agitator 2 and is constituted bythe association of 8 plates 50 together forming a plurality of crosses,defining 25 channels 51 allowing for the passage of the fluid leavingthe tube 3.

In reference to FIGS. 8 and 9, this flow distributor can be constitutedby a number of plates other than 8, for example 4 (FIG. 8) or 9 (FIG.9).

The ratio between the height H of the plates 50 and their maximum widthB is preferably between 1.5 and 2.

The invention claimed is:
 1. A method of treating wastewater including:a. measuring a concentration of pollutants in the wastewater to form ameasurement of pollutants in the water; b. directing the wastewater to aballasted flocculation system; c. forming a sludge-ballast mixture inthe ballasted flocculation system; d. separating ballast from thesludge-ballast mixture and recirculating the separated ballast into theballasted flocculation system; e. determining, utilizing the measurementof pollutants in the wastewater, a ballast concentration level that isrequired in the system to produce a treated water of a predeterminedquality; f. measuring the ballast concentration level in the ballastedflocculation system; g. when the ballast concentration level is lessthan the required ballast concentration level, calculating an additionalamount of ballast required to be added to the system in order toincrease the ballast concentration in the system to the required level;and injecting the required additional amount of ballast into the systemand h. wherein the additional amount of ballast injected into theballasted flocculation system is in addition to the recirculated ballastdirected back to the ballasted flocculation system.
 2. The method ofclaim 1 including continuously repeating two or more of the followingsteps: a. measuring the concentration of pollutants in the wastewater;b. determining the ballast concentration level that is required in thesystem to produce treated water of the predetermined quality; c.measuring the ballast concentration in the system; d. when the ballastconcentration level is less than the required ballast concentrationlevel, calculating the amount of ballast required to be added in orderto increase the ballast concentration in the system to the requiredlevel.
 3. The method of claim 1 wherein recirculating ballast includesdirecting the sludge-ballast mixture from a settling tank to anintermediate tank and directing the sludge-ballast mixture from theintermediate tank to a sludge separation device and separating ballastfrom the sludge-ballast mixture and recycling the separated ballast tothe ballasted flocculation system.
 4. The method of claim 1 includingdirecting wastewater to a flocculation tank, directing ballast from asettling tank, injecting ballast from two or more sources into aflocculation tank, and mixing the injected ballast with the wastewater.5. The method of claim 4 wherein mixing the ballast with the wastewaterincludes directing the wastewater into a peripheral zone of theflocculation tank, directing the wastewater and injected ballast into aninterior zone within an immersed flow-guide structure; injecting aflocculating agent into the wastewater; and, mixing the wastewater andinjected ballast with the flocculating agent to form a water-flocmixture.
 6. The method of claim 5 including directing the water-flocmixture generally downward in the interior zone and out of the interiorzone near bottom portion thereof, directing the mixture generally awayfrom the bottom portion and thence generally upwardly through theperipheral zone, and directing at least a portion of the mixture into anupper end portion of the interior zone and generally downwardly therethrough.
 7. The method of claim 3 including directing the sludge-ballastmixture from the settling tank to a sludge-ballast separating unit,separating ballast from the sludge-ballast mixture, and injecting theseparated ballast to a flocculation tank.
 8. The method of claim 7wherein injecting the separated ballast to the flocculation tankincludes directing an underflow of a hydrocyclone to the flocculationtank.
 9. The method of claim 8 including forming an overflow of thehydrocyclone wherein the overflow comprises a diluted sludge, directingthe diluted sludge to an accumulation tank, discharging a first portionof the diluted sludge from the accumulation tank, and intermittentlydirecting a second portion of the diluted sludge to the intermediatetank and mixing the diluted sludge with the sludge-ballast mixture. 10.The method of claim 1 including calculating the required ballastconcentration level continuously, utilizing the required ballastconcentration level in continuously calculating the concentration of aflocculating agent required to obtain treated water of the predeterminedquality, and, in accordance therewith, continuously adjusting an amountof flocculating agent injected.
 11. The method of claim 10 includingcontinuously injecting a coagulating reagent and a pH-adjusting reagentinto the wastewater.
 12. The method of claim 11 including continuouslyinjecting one or both of the coagulating reagent and the pH-adjustingagent into the wastewater prior to directing the wastewater into thesystem.
 13. A method of treating wastewater using a ballastedflocculation process comprising: a. measuring at least one property ofthe wastewater and determining a concentration of one or more pollutantsin the wastewater; b. calculating, based on the concentration of one ormore pollutants in a wastewater, a required ballast concentration levelin a flocculation tank to obtain a predetermined quality of the treatedwater; c. directing the wastewater into the flocculation tank andinjecting a ballast and a flocculating reagent into the wastewater; d.forming a water-floc mixture in the flocculation tank, the water-flocmixture comprising ballasted flocs; e. directing the water-floc mixtureinto a settling tank, settling at least some of the ballasted flocs, andforming a sludge-ballast mixture in the settling tank; f. directing atleast a portion of the sludge-ballast mixture from the settling tankinto an intermediate tank and therein agitating the sludge-ballastmixture; g. directing the sludge-ballast mixture from the intermediatetank to an inlet of a hydrocyclone; h. using the hydrocyclone toseparate ballast from the sludge-ballast mixture and recirculating theseparated ballast by directing the separated ballast into theflocculation tank; i. determining a ballast concentration level in theflocculation tank and determining whether the ballast concentrationlevel in the flocculation tank is sufficient to obtain the predeterminedquality of the treated water; j. if needed, calculating an additionalamount of ballast required to be injected to increase the ballastconcentration level in the flocculation tank to a level sufficient toobtain the predetermined quality of treated water; and, k. if needed,injecting the amount of additional ballast into the flocculation tankwith the recirculated ballast.
 14. The method of claim 13 includingcontinuously repeating two or more of the following steps: a.calculating the required ballast concentration in the flocculation tankto obtain the predetermined quality of the treated water; b. determiningthe ballast concentration in the flocculation tank and determiningwhether the ballast concentration in the flocculation tank is sufficientto obtain the predetermined quality of the treated water; c. calculatingan amount of additional ballast required to increase the ballastconcentration in the flocculation tank to a level sufficient to obtainthe predetermined quality of treated water; and, d. injecting the amountof additional ballast into the flocculation tank.
 15. The method ofclaim 13 including discharging a first portion of a diluted sludgeoverflow from the hydrocyclone and selectively directing a secondportion of the diluted sludge overflow from the hydrocyclone to thesludge-ballast mixture in the intermediate tank.
 16. The method of claim15 wherein selectively directing the second portion of the dilutedsludge overflow to the sludge-ballast mixture in the intermediate tankincludes determining the level of the sludge-ballast mixture in theintermediate tank, opening a valve to direct the second portion into theintermediate tank while the level of sludge-ballast mixture in theintermediate tank is lower than a pre-determined value, and closing thevalve upon the level reaching the predetermined value.
 17. The method ofclaim 13 wherein determining the ballast concentration in theflocculation tank includes measuring a ballast concentration in thesludge-ballast mixture and calculating the ballast concentration in theflocculation tank using a known proportionality.
 18. The method of claim13 wherein calculating the required ballast concentration in theflocculation tank to obtain the predetermined quality of the treatedwater includes utilizing the equation:Y=aX ^(b) +C wherein Y is the ballast concentration necessary to obtaintreated water of a predetermined quality, a is between about 0.4 and 1,b is between 0.3 and 1, C is between 0 and 2, and X is measuredconcentration of one or more pollutants in the wastewater.